1. Field of the Invention
The present invention relates to an apparatus and an improved process for recycling used glycol-based engine coolant.
2. Description of Related Art
Engine coolants are used to protect internal combustion engines from temperature extremes. The coolants employ ethylene glycol with or without propylene glycol to lower the freezing point and raise the boiling point of water in internal combustion engines. These coolants also commonly employ corrosion inhibitors to counteract the corrosion associated with engine coolants. The coolants typically contain ethylene glycol, with or without propylene glycol, at a concentration of about 30 to 70% by volume, and corrosion inhibitors at approximately 1 to 3% by weight.
The American Society of Testing Materials (ASTM) sets recognized industry standards for the components of engine coolants, both virgin and recycled. The Maintenance Council (TMC) of the American Trucking Association also sets recognized standards for engine coolants.
The useful life of engine coolants is limited by the degradation over time of the glycol and inhibitor components. The glycol components (ethylene glycol with or without propylene glycol) chemically break down into various undesirable organic acids. The accumulation of these acids lowers the pH of the engine coolant composition from an initial pH of approximately 10. When the pH decreases below approximately 8.3, aluminum, steel and iron corrode, weakening the components that they comprise and resulting in eventual component functional failure. In heavy-duty applications, the necessary and commonly practiced addition of supplemental coolant additives (SCAs) increases the total dissolved solids (TDS) in the coolant until a saturation threshold is approached, at which point severe engine damage is probable. Coolant with excessive TDS is associated with premature failure of radiators, water pumps and other cooling system components. Copper and brass also corrode upon extended exposure to engine coolant. In addition, the lead solder in radiators can degrade into lead oxide, eventually allowing leaks to develop in the coolant system heat exchangers.
Because of the well-established corrosive effects of the circulating coolant, initial corrosion inhibitors and SCAs are conventionally added directly to the engine coolant. However, with time the inhibitors are consumed, allowing the corrosive effects of the engine coolant to damage the entire cooling system. Implementation of certain maintenance procedures is necessary to prevent such damage. The most common procedure is removal and replacement of the engine coolant after a pre-established time or service interval. In some instances cooling system additives, which are usually alkaline and include SCAs, are directly added to the coolant to extend the protective properties, decrease the corrosive effects, and postpone replacement of the coolant.
Recycling used engine coolant has become increasingly desirable due to two significant factors. First, engine coolant frequently merits designation as a hazardous waste under the Federal Clean Water Act. Federal and some state environmental protection agencies have instituted strict regulation of the disposal of used engine coolant. In some cases the disposal of engine coolant requires imposition of waste disposal fees and surcharges. Second, ethylene glycol, the principal cost component of engine coolant, has become more expensive. Dramatic price fluctuations and significant shortages in the supply of ethylene glycol have occurred. Therefore, there are both environmental and economic pressures to recycle engine coolant and recover the glycol component in an efficient and cost-effective manner.
A number of processes and systems have been advanced as possible commercially viable solutions to the problem of recycling engine coolant. Most of the processes have employed filtration, ion exchange or distillation techniques in various forms. Distillation of used engine coolant alone sometimes produces an acceptable end product; however, it is an expensive, energy consumptive, and relatively slow process which is not suitable for many potential users. Furthermore, distillation equipment is capital intensive and is associated with various workplace hazards. Relatively simple chemical and filtering techniques have been employed apart from and in conjunction with distillation for recovering and recycling used engine coolant. Engine manufacturers have required more thorough purification of the fluid than can be accomplished through simple or chemical filtration methods. This requirement has been underlined in light of advancements in antifreeze chemical inhibitor technologies, and requirements that all pre-existing inhibitor chemistries be removed from the used engine coolant have been imposed.
An early system for recycling engine coolant used "chemically assisted filtration." The concept of this system was that aeration and pH adjustment of coolant would precipitate contaminants, and the addition of chemical additives would restore adequate functionality to permit extended use of the coolant. U.S. Pat. No. 4,946,595 discloses a process for physically and chemically treating used engine coolant of a type which contains one or more glycol and/or alcohol based antifreeze components. The disclosed process includes the steps of oxidation with one or more known oxidizing agents, precipitation with one or more known salt forming agents, and filtration. Chemical additives are added to enhance the oxidation and precipitation. Various corrosion inhibitors and buffering agents are also added to adjust the pH of the recovered solution. However, the appearance of coolant recycled by chemically assisted filtration was commercially unacceptable in many cases. There were also concerns that filtration alone did not provide adequate purification of the recycled coolant.
Another method available for recycling engine coolant is the reverse osmosis (R/O) process, which provides an intermediate fluid that is pure and offers a production rate that is fairly cost-effective compared to distillation (Huff 1992 SAE 921635). The osmosis phenomenon is observed in nature as a distribution or equalization behavior of naturally occurring chemicals. The R/O process for recycling engine coolant has been described in detail in the literature (Eaton et al., 1997 SAE 971773). U.S. Pat. No. 5,167,826 discloses an apparatus and a process for purification of engine coolant by reverse osmosis, and a process for reinhibition with anti-corrosion inhibitors and buffering agents common to antifreeze manufacturing. Reverse osmosis is a faster and more economical method of coolant recycling than distillation, and provides far better purification than filtration.
There are, however, shortcomings to the R/O process for recycling used engine coolant described in U.S. Pat. No. 5,167,826. In particular, when oil or similar petroleum products enter the RIO system, they coat the membranes, "blinding" them, and reducing production of recycled coolant to a trickle. In addition, recycling engine coolant by the R/O system alone does not decrease conventional inhibitor anions in the coolant to a level that would permit reinhibition of the intermediate fluid either with traditional, inorganic technologies, or European style carboxylate inhibitors.
Thus, there has been a need for a way to provide a highly purified recycled engine coolant at a cost advantageous compared to virgin coolant.